The ancient Aya Sofya, often called Hagia Sophia in English and one of Istanbul’s most iconic and enduring monuments at nearly 15 centuries old, has been converted from a museum back into a mosque with a recent decree from Turkey’s president, Recep Tayyip Erdoğan. The building has been classified as a museum since 1934 after a declaration from Mustafa Kemal Atatürk, the first Turkish president and founder of the Republic of Turkey. The structure, considered a jewel of Byzantine architecture, was originally built as a cathedral in 537 CE by Roman Emperor Justinian I. Aya Sofya also had an almost five-century stint as a mosque when the Ottomans conquered Istanbul, then called Constantinople, in 1453 and converted the building into a religious space for Muslims.

İstanbul's most photogenic building was the grand project of Sultan Ahmet I (r 1603–17), whose tomb is located on the north side of the site facing Sultanahmet Park. The mosque's wonderfully curvaceous exterior features a cascade of domes and six slender minarets. Blue İznik tiles adorn the interior and give the building its unofficial but commonly used name.

The Byzantine emperors loved nothing more than an afternoon at the chariot races, and this rectangular arena alongside Sultanahmet Park was their venue of choice. In its heyday, it was decorated by obelisks and statues, some of which remain in place today. Re-landscaped in more recent years, it is one of the city's most popular meeting places and promenades.

Topkapı is the subject of more colourful stories than most of the world's museums put together. Libidinous sultans, ambitious courtiers, beautiful concubines and scheming eunuchs lived and worked here between the 15th and 19th centuries when it was the court of the Ottoman empire. A visit to the palace's opulent pavilions, jewel-filled Treasury and sprawling Harem gives a fascinating glimpse into their lives.

The colourful and chaotic Grand Bazaar is the heart of İstanbul's Old City and has been so for centuries. Starting as a small vaulted bedesten (warehouse) built by order of Mehmet the Conqueror in 1461, it grew to cover a vast area as lanes between the bedesten, neighbouring shops and hans (caravanserais) were roofed and the market assumed the sprawling, labyrinthine form that it retains today.